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# Stepper Motor voltage spec

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#### joe_1

##### Member
Hello,
I am having difficulty understanding stepper motor spec. I have questions about it.
I see a spec like this:

### NEMA 17 Stepper Motor Technical Specifications​

• Rated Voltage: 12V DC
• Current: 1.2A at 4V
Why do they give Current value @ 4V instead of the 12V?
Is this (1.2A) the maximum Current, and will it decrease with higher voltage ?
Will the motor really work with only 4V?
What will happen if I supply 24V? Will it still use 1.2A ?

The important figure is the rated current, which should be maintained all through the RPM range.
A stepper motor inductive reactance increases with RPM, if the 4v supply was maintained the rpm will drop drastically, due to decrease in current, in order to maintain the rated current, the voltage is increased according by the drive, in order to maintain this constant rated current.

It needs a current limiting circuit.

That can be as simple as a series power resistor to set the current.
That wastes a lot of power and heat though, so most modern stepper motor driver ICs use PWM to continuously adjust the current and keep it to whatever has been set for that specific motor.

The motor coils are inductors. When power is switched on to a coil/inductor, the current starts at zero and builds up at some as the magnetic field increases.

Running at 4V would only allow the motor to run very slowly, as the current takes significant time to build at each step.

Using a higher voltage and a current limiting circuit makes the current built much faster and allows the motor to operate correctly.

The faster you need the motor to run, the more important that is, as it has less time between steps so without the higher voltage the torque will rapidly drop to zero as you try to increase the speed.

It drops off with speed regardless, but at much higher speeds with a suitable voltage and drive circuits.

The torque of a stepper motor starts at maximum at zero rpm when the plate rated current & voltage is applied.
As can be seen on the motors specification details.
Running at the plate rated current, the motor will start at maximum (rated) torque decrease speed as rpm increases due to increase in inductive reactance.
Ref: Stepping Motor Design Guide: Superior Electric. Slo-Syn.

So, here are two cases:
Case1: I supply 1A/12V
Case2: I supply 0.5A/24V

Will the torque be the same in both, since I use the same power ?

Will the torque be the same in both, since I use the same power ?
No.

Either setup would need a current limiting device of some sort, otherwise current would just be
(Voltage / Motor_DC_Resistance)

For it to take 1.2A @ 4V, the DC resistance must be around 3.3 Ohms.

At 1A (steady state) it would have 3.3V across it.

At 0.5A it would have around 1.66V across it.
That's a quarter of the power, as both current and voltage are halved.

Yes I have current limiting device with a current sense resistor, so I can control the current precisely.

Thanks

Any decent quality stepper motor spec will show the required DC applied voltage required in order to create the rated continuous operating current.
Maintaining this current is essential when using drives that use the higher voltage methods.

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